1. Field of the Invention
The present invention relates to a voltage regulator including a leakage current sink circuit capable of suppressing a leakage current of an output transistor at high temperature, and reducing power consumption of the voltage regulator at normal temperature.
2. Description of the Related Art
FIG. 6 illustrates a related-art voltage regulator configured to suppress a leakage current of an output transistor. The related-art voltage regulator includes a reference voltage circuit 103, a differential amplifier circuit 104, an output transistor 105, a voltage divider circuit 106, and a leakage current sink circuit 107.
The differential amplifier circuit 104 compares a reference voltage VREF output from the reference voltage circuit 103 and a feedback voltage VFB output from the voltage divider circuit 106, and controls a gate voltage of the output transistor 105 so that an output voltage VOUT of an output terminal 102 is kept at a predetermined value.
The output voltage VOUT is independent of a power supply voltage and is constant as expressed by Expression (1).VOUT=(RS+RF)/RS×VREF  (1)where RS represents a resistance value of a resistor 122, and RF represents a resistance value of a resistor 121.
In a state in which no load is connected to the output terminal 102 or a light load is connected thereto, the differential amplifier circuit 104 controls a gate-source voltage of the output transistor 105 so that the output transistor 105 enters a substantially off state, to thereby cause only a current necessary for keeping an output of the voltage divider circuit 106 to flow, or cause a current obtained by adding to the current a current amount for the light load to flow. In this case, a current Ifb that flows through the voltage divider circuit 106 is ideally expressed by Expression (2).Ifb=VREF/RS  (2)The output voltage VOUT is expressed by Expression (3) with use of the current Ifb flowing through the voltage divider circuit 106.VOUT=(RS+RF)×Ifb  (3)However, at high temperature, a leakage current Ileak of the output transistor 105 flows. The leakage current Ileak exponentially increases along with an increase in temperature to be non-negligible. Thus, in a state in which no load is connected to the output terminal 102 or a light load is connected thereto, the leakage current Ileak ultimately flows into the voltage divider circuit 106.
Hence, Expression (3) is transformed into Expression (4) at high temperature.VOUT=(RS+RF)×(Ifb+Ileak)  (4)Therefore, the output voltage VOUT is increased due to an influence of the leakage current Ileak, and the voltage regulator cannot operate normally. To deal with this, the leakage current sink circuit 107 including a depletion type NMOS transistor 111 and an NMOS transistor 112 is used to reduce the influence of the leakage current (for example, see Japanese Patent Application Laid-open No. 2012-226421).
However, the related-art voltage regulator has a problem in that current flows through the leakage current sink circuit 107 from the output terminal 102 even at normal temperature, and hence the power consumption cannot be reduced.